After a lot of experiments, here is the Ambassadors Combination i like the most. With this combination I got every unlock and achieviement in the game, and also a few that I made for myself (10.000 Prosperity Total, 6 Level 4 Projects in the same planet, 6 cities with more than 2.000 resources in use, and a few others)

Ocean Giant: S-S-D-S: You can get Superior Animais and Sublime Herd Aspect, including some top end animais like Blue Whale, Panda and Bighorn (Panda is just amazing). Even with only one Desert Ambassador, you will get Mosson Level 3. The Fertility Boost helps a lot to reach the Sublime Aspects you will need to reach top end resources. You will be stuck with "Lesser Growth Aspect", so you won't get any useful growth plant (not even Desert Lime), and yes, there are some very good trees that use it. But since you will get Sublime Leaf and Sublime Fruit (see below), it is not a problem, it is a good trade-off.

Forest Giant: F-F-S-F: With this combination, you get Fruit Tree Level 3, Fruit Aspect Level 3 and Leaf Aspect Level 3! Just awesome! Some resources you will get with this: In Deserts, you get Cardon Cactus. In Forest, you get Orange Tree, Plum Tree, Pear Tree and Sunflower. In Swamp, White Willow, Papaya and Rubber Tree. Those resources are more than enough to do anything tree related very, very well.

Mountain: D-S-D-D: With this combination, you will unlock both Minerals as Superiors, and also Sublime Noble Aspect! You can put 2 Swamp instead, and unlock Sublime Seismic, losing Sublime Noble, but I think Reaction is way better for Tech Minerals, not worth sacrificing Sublime Noble for Seismic. And by focusing on Noble, this Giant will be a great money maker! Onyx with powerful natura plants can generate more money than Gold! Diamond and Ruby are simply amazing, and if you mix those with Cardon Cactus, I am sure you will get even the Multinational with out breaking a sweat.

Swamp: D-D-F-F: With this combination, the Swamp Giant won't get any Superior Resource. But it doesn't matter, he will have two awesome Sublime Aspects! His plants aren't that great (you can get food and tech with the Fruit Trees as base for transmutation without any problem) and there isn't any top tier Exotic Animal that is a game changer, imho. Swamp Giant main purpose in the early/middle game is to keep greed in check with danger (that's why i get 2 Deserts right away, so the predator aspect will be at least "potent", otherwise you will have to keep boosting the fertility in the patchs to add danger). And after he gets 2 Forest, he is simply a beast! You will have Copper, Iron, Uranium, Fluroite and Aluminium! Even the 1250 tech needed for the Laboratory is very easy to get, specially if you have Uranium.

Maybe the only achievement or unlock that would be tricky to get with this combination is the Ocean Towns related ones. It is doable, and not that hard, no doubt, but since some fishs need the "Hunt" or "Exotic" aspect, you will be stuck with fewer kinds. Instead of AnglerFish or Marlin, you will get only Tuna, for example. And forget about Sharls, but you will get only the majestic Blue Whale.

And remember: you can get 2 Giants with 4 Ambassadors each without completing any 4th tier Project (Factory, Opera House, Hospital, Town Canal, Castle, Laboratory, etc). So plan ahead and give the 4th ambassadors to the giants that will make good use of it! If you want to complete a Laboratory, Mountain and Swamp would both be awesome for Advanced Minerals and Reaction. If you want to complete a Castle, Superior Domestic Animais and Superior Fruit Plants are the way to go. If you need Awe, Panda, Dragon Fruit and Fluorite goes very well together.

Don't complete any project just because it is there. Prioritize, see if you will benefict more from a Desert, a Forest or a Swamp Ambassador next. Have this in mind, since completing a forth 3rd tier project means that the next ambassador will have to come from a last tier project, and this can be very tricky with out Superior Resources and Sublime Aspects.